Cavity Preparation for Amalgam Restoration Class 2
It is smooth surface lesion occurring on the proximal surface of molars and premolars just gingival to contact area.
1st: Outline form
1-The classical class 2 cavity has two portions:
(1) The occlusal cavity.
(2) The proximal cavity: which meet together at an area called isthmus portion (weakest part it’s above the axial wall).
2- The occlusal cavity similar to class 1 except that it has reverse curve for:
(1) Be away from contact area.
(2) Cavosurface angle=90
3- Proximal cavity location: at contact area to make it free and its gingival wall apical to contact area by 1-0.5 mm. It's CSA=90.
1- occlusal cavity : similar to class
2- proximal cavity:
(1) Buccal and lingual walls parallel to corresponding surface and converge occlusally and flaring proximally.
(2) Gingival wall apical to contact area by 1-0.5 mm.
(3) Axial wall should be converge occlusally and convex bucco-lingually.
(4) Axio-pulpal line angle should be rounded.
(5) Gingival wall should be beveled apically.
(6) Pulpal floor should be parallel to occlusal plane except in the 1st premolar.
2nd: Resistance form:
1- Restoration should be slightly diverge occiusally mesio-distally to give bulk to amalgam.
2- Undermined enamel and short enamel rods must be removed.
3- Flat pulpal and gingival floor parallel to occlusal plane and parallel to each other and perpendicular to tooth long axis.
4- Width of gingival floor (deep of axial wall) is limited to 1mm in premolars and 1.5 nun of molars under DEJ 0.5 mm in premolars and 1 mm of molars.
5- All walls are smooth and flat.
6- Line angels slightly rounded.
7- Axial wall inclines occlusally and convex bucco-lingually. (so we stay away from pulp horn).
8- Rounded or beveled axio-pulpal line angel (to prevent stress concentration of forces at isthmus portion in order to prevent fracture of restoration).
9- CSA occlusally and proximally 90 dgree.
10- Axial wall is straight or inclined occlusally gingivo-occlusally.
11- The proximal preparation should have a mesiodistal dimension of about 1.5 mm or more.
12- If there is sound dentin supporting occlusal enamel in the fossa adjacent to the marginal ridge, that dentin and enamel should be left intact.
3rd: Retention form:
(1): occlusal portion:
1- Buccal and lingual walls converge occiusally (prevent occlusal displacement).
2- Dove tail lock (prevent proximal displacement).
3- Undercuts (prevent occlusal displacement without undermining enamel).
(2): B- proximal portion: By cutting 2 retention proximal axial grooves.
1. If the extension into the occlusal surface is narrower, or if there is no extension into the occlusal grooves, as with the proximal slot restoration, retentive undercuts (retention grooves or points) must be cut into the dentin of the facial and lingual walls of the proximal box
2. For a proximal slot restoration, retentive undercuts should be very distinct (at least 0.5 mm deep) and should oppose each other to form a dovetail effect in the dentin.
3. Long grooves, extending from the gingival floor to the occlusal surface, are rec¬ommended for a proximal slot restoration.
4. If the occlusal extension is narrow, short retention grooves, or retentive points, should be prepared in the dentin of the facial and lingual walls to supple-ment the resistance form provided by the occlusal extension.
5. If there is a bulky extension of amalgam into the occlusal surface of the tooth, reten¬tive undercuts should not be necessary.
6. Retentive undercuts in the dentin of the facial and lingual walls should be completely in dentin and not at the DEJ.
7. A good rule is to place retentive undercuts so that there is approximately 0.25 to 0.5 mm of dentin between the groove and the DEJ and so that the groove is approximately 0.5 mm deep and 0.5 mm wide.
4th: planning of enamel walls (Noy’s principles):
A- Enamel walls finished from any short or undermined enamel.
B- Enamel walls meet tooth surface at right cavosurface angel.
C- All sharp corners in enamel must be rounded.